1. Field of the Invention
The present invention relates to a drum-type washing machine, and more particularly, to a drum-type washing machine having a self-balancing outer tub assembly.
2. Discussion of the Related Art
Referring to FIG. 1, the exterior construction of a drum-type washing machine according to a related art includes a cabinet frame 2, a cabinet front cover 4 providing the cabinet with a forward-facing opening 4a in which a door assembly 5 having a view port is installed, and a base assembly 8 on which the cabinet frame is supportably mounted. A top plate assembly 6 is mounted atop the cabinet frame 2, and a control panel assembly 7, provided with user interface features, is installed at one end of the top plate assembly 6. The door assembly 5 provides access to the interior of the drum-type washing machine.
Referring to FIG. 2, showing the interior construction of a drum-type washing machine according to a related art, a drum 10 for holding laundry is rotatably installed within a cylindrical outer tub for supporting the rotating drum and laundry. The outer tub is typically manufactured by injection molding of a synthetic resin-based material and may be integrally formed as one body or, to enable manufacture using smaller molds, may be constituted as front and rear outer tubs 12 and 14, as shown in the drawing. The front and rear outer tubs 12 and 14 are generally formed of the same material, e.g., a polypropylene containing about 25% glass fiber or a polypropylene containing about 10% glass fiber and about 17% CaCO3, and are tightly coupled to each other to create a watertight junction.
Notably, the constitution of the rear outer tub 14 is fundamentally different from that of the front outer tub 12. For example, the rear outer tub 14 has one end closed, save a through-hole 14a for receiving a rotational shaft (not shown) on which the drum 10 is rotated, and is inherently heavier than the front outer tub 12, which has an open front and rear. Moreover, a plurality of reinforcing ribs 14b is formed at the rear of the rear outer tub 14, to withstand vibrations generated when the drum 10 is rotated. It should be appreciated that the fundamental differences between the front and rear outer tubs 12 and 14 would be present even in the case of an integrally formed outer tub assembly. In either case, the weight distribution of the cylindrical outer tub results in its forward part being much lighter. That is, in spite of being formed of the same material, the front and rear outer tubs 12 and 14 exhibit a substantial difference in weight, due to their contrasting structures and functions.
Referring to FIG. 3, the constructional differences between the front and rear outer tubs 12 and 14 become more apparent. For example, in a direct drive system in which the motor assembly (not shown) is mounted on the rear side of the rear outer tub 14 at the drum's central axis, a pair of bearings 14c are installed within the through-hole 14a to support the rotational shaft, and a spider (not shown) is provided to disperse the forces of a shear stress transferred to the rear outer tub 14 during drum rotation. The motor assembly consists of a stator assembly and a rotor assembly.
The weight difference between the front and rear outer tubs 12 and 14 brings about shaking and noise when the drum 10 is rotating. Therefore, a counterbalance 16 for maintaining the balance of the front and rear outer tubs 12 and 14 is installed at one end of the front outer tub 12, namely, the forward end, to counteract the forces and effect of the rotating drum 10. While the counterbalance 16 is typically mounted to the front exterior of the tub assembly, its precise positioning is determined by the characteristics of the washing machine, and in particular, by the weight distribution of the rear outer tub 14. In any event, the counterbalance 16 is positioned in direct opposition to the weight imbalance of the rear outer tub 14.
The counterbalance 16 may be realized by one of several means, including a ring-type counterbalance fixed in two parts to the front outer tub 12, as shown in the drawings, using a coupling means such as a plurality of screws. Such a counterbalance is achievable by molding a dense cement or by filling with cement the interior of a synthetic resin-based casing having an opening for inserting the cement.
In a drum-type washing machine according to the related art, however, the number of manufacturing components is increased, which substantially increases the part count and increases assembly time accordingly. Besides, the counterbalance itself is a very heavy, cumbersome component. Moreover, the cement of the counterbalance generates cement powder and other contaminants and therefore deteriorates the manufacturing environment and increases the likelihood of operational malfunction by introducing such contaminants to the interior of the manufactured product.